Paper Coating Lubricants
Lubricants are used in the coatings of paper and board for a number of reasons. They improve the runnability of the coating color by reducing the friction between the machine and the coating color, and also by reducing the friction between the base paper and the coating unit. This can be seen for example as fewer scratches in the coating and longer lifetime of coater blades. They enhance the plastic deformation of the dry coating in the supercalender by preventing the cracking of the soluble binder film that leads to dusting. This also improves gloss.
During calendering, lubricants migrate from the coating onto the hot calender rolls forming a monolayer on the rolls and thus preventing the sticking of the coating to the rolls that causes build-ups. Different kaolins have different dusting tendencies, and dusting can often be solved by using a lubricant. The most commonly used lu¬bricant is calcium stearate. It is produced by reacting stearic acid with calcium hydroxide, followed by emulsification and processing to a 50 % aqueous disper¬sion. The critical properties of the calcium stearate dispersion are mechanical im¬purities and free Ca2+, a good dispersion contains a minimum amount of both.
Particle size and shape are important for antidusting properties. The optimum size is 5–10 mm and the shape should be platy. The platy shape enables the stearate to gather on the pigment, plasticize the surface of the dry coating, and reduce the dusting at calenders and printing machines. Wax emulsions are mostly emulsions of paraffin waxes, microcrystalline waxes, or polyethylene waxes. These are the oldest group of lubricants in paper and board coating. These emulsions give good runnability but have less effect as antidusting agents than does stearate. Particle size is small and the dry solids content of the emulsion is usually 20 %-30 %. Soy lecithin/oleic acid blends are a new group of substances used as lubricants. Poly¬ethylene and polypropylene glycols are used in blends with calcium stearate or alone as a lubricant. These lubricants are claimed to have influence on the rheology and flow properties of the coating color.